Gas-assisted injection molding is generally the preferred method for the production of large and complicated parts without sink marks and the like in thick rib sections or complicated cross-sectional areas and for production of parts where weight reduction is important. Such parts include plastic cladding and body mold parts designed for the automobile industry. In gas-assisted injection molding these large and complicated parts are made of a hollow construction by injecting a gas into the interior of a part while it is being formed in a mold cavity. The gas injected into the interior of the mold cavity will follow the so-called "path of least resistance." As the molten plastic cools and hardens from the outside inwardly, the gas will penetrate the inner and thicker portions of the part which are generally softer and warmer. Thus hollow cavities are generally formed in the thicker sections where, for example, structural ribs and the like may be located. The pressurized gas also expands these hollow cavities thereby forcing the molten plastic outwardly to fill the mold cavity. If the gas pressure is maintained at a relatively high level while the plastic part cools, the surface finish of the plastic part will be greatly enhanced by the minimization or elimination of sink marks, depressions, or like imperfections. Such hollow cavities can also result in considerable weight savings in the finished plastic part. Prior to the present invention the role of the gas channels was generally limited to enhancing the surface appearance and/or weight reduction in the finished part.
The nominal diameter of the gas channels in prior art plastic parts such as plastic claddings and body moldings were generally less than about 0.25 inches. Such plastic claddings and body moldings in the prior art were a two-part plastic assembly. The outer surface of the plastic cladding or body mold parts was shaped to the desired configuration. Attached to that outer part--usually by heat staking or sonic welding--was an inner plastic part designed to fit within and bond to the outer plastic part and then to be affixed to the desired surface of the automobile. The actual attachment was obtained by bonding the inner plastic part to the automotive surface with metal or plastic fasteners, friction fitting, or adhesive systems. Thus, the prior art systems required the design and production of two injection mold cavities, the injection molding of two plastic parts, and the assembly of the inner and outer plastic parts. In addition to increased costs and complexity in the production process, the requirement of a two-part assembly results in increased weight of the assembled part which not only adds to the total weight of the resulting automobile but also increases the demands on the attachment system used. In some cases, the increased weight may become so large that the available attachment systems cannot be used; thus, there may be a design limitation on the resulting plastic cladding and body molding parts. As those skilled in the art will realize, a two-part assembly will generally be weaker and more prone to failure than a monolithic, one-piece part of similar design and outward appearance. In addition, the points of attachment of the two plastic parts in the assembly (i.e., the bonding of the inner and outer plastic parts together) represent additional potential failure points. Such failures in the plastic cladding and body molding parts in the automotive market can have a significant, immediate, and negative impact on consumers' perception of the quality and quality control of the particular automobile and its manufacturer.
It is desirable, therefore, to produce plastic cladding and body molding parts, especially for automobile applications, which are of monolithic, one-piece construction and which are suitable for, and designed for, direct attachment to the substrate surface. It is also desirable to produce such plastic cladding and body molding parts at a weight savings over comparable two-part plastic assemblies. It is also desirable to produce plastic cladding and body molding parts which are stronger and less prone to failure than the conventional two-part plastic assemblies currently in use. The present invention provides such plastic cladding and body molding parts. The plastic cladding and body molding parts of the present invention are especially adapted and suitable for use in automotive applications. These plastic cladding and body molding parts can be used in the interior or exterior of automobiles as well as in many other applications.